Plastic pipe, particularly polyvinylchloride (PVC) pipe, has become widely accepted in many applications in place of or as a substitute for metallic pipe. In the installation of a plastic pipe, it may be desirable to change its configuration. For example, it may be necessary to bend the pipe to permit it to be passed over or around an obstruction. In many instances, particularly in the installation of pipe underground, the presence of such obstructions may not be known in advance. When such an obstruction is discovered, such as during the digging of a trench, it is desirable to have some means available for bending the pipe at the installation site to permit it to pass around the obstruction.
Accordingly, a need exists for apparatus which can be used at an installation site and moved from one site to another for bending plastic pipe. A number of techniques have been employed in the past for heating plastic pipe at the installation site to permit it to be molded to a desired configuration. However, such techniques have been only partially successful and generally have one or more disadvantages.
One such technique employs an electric blanket or pad which can be wrapped around the exterior of the plastic pipe to raise the temperature of that portion of the pipe sufficiently to permit it to be molded. Of course, such a heating element requires a source of electrical power which is generally not available at an installation site. In addition, the heat transfer time is generally long and only a small portion of the pipe can be heated at any one time. Accordingly, the use of an electric blanket or pad for heating a plastic pipe, if electrical power is available, is generally slow and time consuming. Another technique which has been employed is that of raising the temperature of the plastic material by the direct application of a flame to the external surface of the pipe. This process, of course, can only heat a relatively small surface area of the pipe at any given time and is, therefore, a relatively slow and tedious process. Furthermore, the direct application of a flame, such as by the use of a blow torch, to the external surface of a plastic pipe, tends to burn the outer layer of material, thereby reducing its strength and other qualities.
Both of the above mentioned techniques for heating a plastic pipe suffer from the disadvantage of not having a medium which retains heat from the heat source and which can be employed for transferring heat to the plastic pipe. That is, each of the above techniques has no means for storing heat and transferring that stored heat to the pipe. Accordingly, these techniques are generally slow and time consuming and, therefore, undesirable.
Another technique employs an electric quartz heater in which the quartz is used as a heat source for retaining and transferring heat to the plastic pipe. Although the electric quartz heater does not suffer from the disadvantage mentioned in connection with the other techniques, all of the mentioned techniques apply heat only to the external surface of the pipe. When heat is applied to only an external surface of the pipe, its moldability is reduced. The electric quartz heater also suffers from the disadvantage of requiring a source of electrical power at the installation site.
Accordingly, it can be readily appreciated that a need exists for apparatus for heating plastic pipe to permit it to be molded to any desired configuration, which is portable, requires no electrical power, provides heat to both the external and internal surfaces of the pipe, heats the pipe at a relatively fast rate, and does not damage the pipe or reduce its strength and other characteristics.